No, it's actually quite simple...in summary:
For mipspro 7.3(any compiler), you need the Dev. Foundation 1.2 CD aswell as the compiler cd, eg. MipsPro C/C++/F77/F90.
With regard to you "really" wanting the C++ compiler...do you compile any C++ code? If you don't than just the C compiler cd is fine. So I don't see how the C compiler cd "won't do you any good".
GCC is fine for most things, but some software just won't compile with GCC on irix machines...so it can be usefull to have a MipsPro compiler around as well.
You best bet is to wait for a full 7.3 compiler set to come up for auction..then you'll have what you need. I went thru the problems of just getting the C compiler cd for 7.3 with a batch of older 7.2.1 cd for cheap, and then couldn't install 7.3 because I didn't have the right version of the Dev. Foundations.
"Two Minus Three Equals Negative Fun" - Troy McClure